1. Field of the Invention
The present invention relates to a spark plug.
2. Description of the Related Art
Japanese Patent Application Laid-Open (kokai) No. 2002-319469 discloses a spark plug in which noble metal tips, containing a predominant amount of a noble metal, are joined respectively to a center electrode and a ground electrode; the noble metal tips have an outside diameter of 0.8 mm or less; and the length of projection of the noble metal tip from the base metal of the ground electrode is 0.3 mm to 1.5 mm. Japanese Patent Application Laid-Open (kokai) No. 2002-184551 discloses a spark plug in which noble metal tips, containing a predominant amount of a noble metal, are joined respectively to a center electrode and a ground electrode; the noble metal tips have an outside diameter of 0.8 mm or less; and the length of projection of the noble metal tip from the base metal of the ground electrode is 0.5 mm to 1.2 mm.
3. Problems to be Solved by the Invention
In the above-disclosed spark plugs, when a portion of the noble metal tip of the ground electrode that projects from the electrode base metal is reduced in diameter in order to enhance ignition performance, the noble metal tip of the ground electrode is consumed to a greater than the noble metal tip of the center electrode. This is because the temperature increase of the noble metal tip of the ground electrode tends to be greater than that of the center electrode. As a result, the noble metal tip of the ground electrode tends to lack durability as compared with the noble metal tip of the center electrode.